In a new algorithm for maintaining replicated data, every copy of a replicated file is assigned some number of votes. Every transaction collects a read quorum of r votes to read a file, and a write quorum of w votes to write a file, such that r+w is greater than the total number number of votes assigned to the file. This ensures that there is a non-null intersection between every read quorum and every write quorum. Version numbers make it possible to determine which copies are current. The reliability and performance characteristics of a replicated file can be controlled by appropriately choosing r, w, and the file's voting configuration. The algorithm guarantees serial consistency, admits temporary copies in a natural way by the introduction of copies of an application system called Violet.

Distributed computing systems are being built and used more and more frequently. This distributod computing revolution makes the reliability of distributed systems an important concern. It is fairly well-understood how to connect hardware so that most components can continue to work when others are broken, and thus increase the reliability of a system as a whole. This report addressos the issue of providing software for reliable distributed systems. In particular, we examine how to program a system so that the software continues to work in tho face of a variety of failures of parts of the system. The design presented

Bayou is a replicated, weakly consistent storage system designed for a mobile computing environment that includes portable machines with less than ideal network connectivity. To maximize availability, users can read and write any accessible replica. Bayou's design has focused on supporting apphcation-specific mechanisms to detect and resolve the update conflicts that naturally arise in such a system, ensuring that replicas move towards eventual consistency, and defining a protocol by which the resolution of update conflicts stabilizes. It includes novel methods for conflict detection, called dependency checks, and per-write conflict resolution based on client-provided merge procedures. To guarantee eventual consistency, Bayou servers must be able to rollback the effects of previously executed writes and redo them according to a global senalization order. Furthermore, Bayou permits clients to observe the results of all writes received by a server, Including tentative writes whose conflicts have not been ultimately resolved. This paper presents the motivation for and design of these mechanisms and describes the experiences gained with an initial implementation of the system.

"From a certain point onward there is no longer any turning back. That is the point that must be reached." Kafka Protocols that allow operational sites to continue transac-tion processing even though site failures have occurred are called nonblocking. Many applications require nonblocking Qrotocols. This paper investigates the properties of non-blocking protocols. Necessary and sufficient conditions for a protocol to be nonblocking are presented and from these conditions a method for designing them is derived. Both a central site nonblocking protocol and a decentralized non-blocking protocol are presented.

A number of recent studies have examined the performance of concurrency control algorithms for database management systems. The results reported to date, rather than being definitive, have tended to be contradictory. In this paper, rather than presenting “yet another algorithm performance study,” we critically investigate the assumptions made in the models used in past studies and their implica-tions. We employ a fairly complete model of a database environment for studying the relative performance of three different approaches to the concurrency control problem under a variety of modeling assumptions. The three approaches studied represent different extremes in how transaction conflicts are dealt with, and the assumptions addressed pertain to the nature of the database system’s resources, how transaction restarts are modeled, and the amount of information available to the concurrency control algorithm about transactions ’ reference strings. We show that differences in the underlying assumptions explain the seemingly contradictory performance results. We also address the question of how realistic the various assumptions are for actual database systems.

Database replication is traditionally seen as a way to increase the availability and performance of distributed databases. Although a large number of protocols providing data consistency and fault-tolerance have been proposed, few of these ideas have ever been used in commercial products due to their complexity and performance implications. Instead, current products allow inconsistencies and often resort to centralized approaches which eliminates some of the advantages of replication. As an alternative, we propose a suite of replication protocols that addresses the main problems related to database replication. On the one hand, our protocols maintain data consistency and the same transactional semantics found in centralized systems. On the other hand, they provide flexibility and reasonable performance. To do so, our protocols take advantage of the rich semantics of group communication primitives and the relaxed isolation guarantees provided by most databases. This allows us to eliminate the possibility of deadlocks, reduce the message overhead and increase performance. A detailed simulation study shows the feasibility of the approach and the flexibility with which different types of bottlenecks can be circumvented.

Voting schemes ensure the consistency of replicated files by disallowing all read and write requests that cannot collect an appropriate quorum of copies. This procedure requires a minimum number of three copies to be of any practical use and tends to disallow a relatively high number of read and write requests. We propose to replace some of these copies by mere records of the current state of the file. These records, called witnesses, will be assigned weights and participate to the collection of quorums. We show, that under very general assumptions, the reliability of a replicated file consisting of n copies and m witnesses is the same as the reliability of a replicated file consisting of n + m copies. We also compare the availability of a replicated file consisting of two copies and one witness with that of a file having three copies and show that, under normal circumstances, the two files have similar availabilities.

Replication is an area of interest to both distributed systems and databases. The solutions developed from these two perspectives are conceptually similar but differ in many aspects: model, assumptions, mechanisms, guarantees provided, and implementation. In this paper, we provide an abstract and “neutral ” framework to compare replication techniques from both communities. The framework has been designed to emphasize the role played by different mechanisms and to facilitate comparisons. The paper describes the replication techniques used in both communities, compares them, and points out ways in which they can be integrated to arrive to better, more robust replication protocols. 1.

This paper deals with the transaction management aspects of the R * distributed database system. It concentrates primarily on the description of the R * commit protocols, Presumed Abort (PA) and Presumed Commit (PC). PA and PC are extensions of the well-known, two-phase (2P) commit protocol. PA is optimized for read-only transactions and a class of multisite update transactions, and PC is optimized for other classes of multisite update transactions. The optimizations result in reduced intersite message traffic and log writes, and, consequently, a better response time. The paper also discusses R*‘s approach toward distributed deadlock detection and resolution.

We describe a new replicated-object protocol designed for use in mobile and weakly-connected environments. The protocol differs from previous protocols in combining epidemic information propagation with voting, and in using fixed per-object currencies for voting. The advantage of epidemic protocols is that data movement only requires pairwise communication. Hence, there is no need for a majority quorum to be available and simultaneously connected at any single time. The protocols increase availability by using voting, rather than primary copy or primary commit schemes. Finally, the use of per-object voting currencies allows votes to take place in an entirely decentralized fashion, without any server having complete knowledge of group membership. We show that currency allocation can be used to implement diverse policies. For example, uniform currency distributions emulate traditional dynamic voting schemes, while allocating all currency to a single server emulates a primary-copy scheme...